If you’re a regular reader of the Loom, you’re no doubt familiar with the parasite Toxoplasma gondii. If you’re not, now is the perfect time to meet this sinister creature which may very well be residing in your brain. It seems like every year or two, it gets more remarkable, and today it’s taken another step into awesomeness.

Here’s a quick Toxoplasma primer. It’s a single-celled protozoan that reproduces inside the digestive tract of cats. The cats poop out egg-like Toxoplasma cells into kitty litter and dirt. Other animals take up the parasite, which makes its way into their tissues, especially the brain. There it forms cysts that can linger for years or decades. Only if that animal gets eaten by a cat can Toxoplasma complete its life cycle.

This life cycle opens up opportunities for Toxoplasma to evolve. For example, natural selection should favor mildness in the parasite in its hosts, because cats do not like to eat corpses. And, indeed, Toxoplasma is fairly harmless, only causing trouble to people with suppressed immune systems. (Hence the rule that pregnant women should not handle kitty litter. If they get infected by Toxoplasma for the first time, the parasite runs amok in the fetus.) On the other hand, if there’s any way for the parasite to increase the odds that it can get from prey to cat, natural selection may favor genes for that strategy too.

And it turns out that Toxoplasma does have that very ability. In studies on rats, scientists have found that infected rodents lose their fear of the scent of cats. In fact–and please remember, I am a science writer, not a Hollywood script doctor–the rats may even become sexually aroused by the smell of cats. They embrace their doom, and the parasite benefits.

These findings have lots of interesting implications for humans, because perhaps a quarter of all people on Earth carry these parasites in their heads, where they no doubt secrete their mind-altering compounds. There’s some preliminary work that suggests some changes to the personality of infected people, but nothing definitive.

That would be enough for Toxoplasma to earn its place in the Parasite Hall of Fame. But, no, it needed to go one better.

It turns out that rats and other non-cat hosts can spread Toxoplasma to each other through sex. The first reports have only just emerged from studies on dogs and sheep. Recently Ajai Vyas, a neuroscientst at Nanyang Technological University in Singapore, decided to see whether rats can spread Toxoplasma the same way. In the journal PLoS One, he and his colleagues describe how they mated infected males with uninfected females. They found Toxoplasma in the male rats’ semen, and, after mating, in the female rats’ vaginas. And later, they found signs of Toxoplasma in the female rat brains.

These are Toxoplasma cysts moving from rat to rat, so this exchange is kind of like a side track on the parasite’s life cycle. But it still benefits Toxoplasma, because it means it can infect even more potential prey that may get eaten by cats. And so the logic applies once more: if Toxoplasma can raise the odds of getting from infected males to uninfected females, it may have more reproductive success.

You know where this is going–it’s turning into a David Cronenberg horror movie with an all-rodent cast. Vyas wondered if there’s any difference in how female rats mate with infected and uninfected males. So he and his colleagues put a male rat with Toxoplasma at one end of a two-armed maze, and an uninfected male in the other arm. Females then got to choose which rat to approach. Vyans found that they preferred the infected males, spending more time with them and mating more often.

In other words, Toxoplasma makes its host sexy, in order to get into other hosts through sex.

As I wrote in Parasite Rex, many parasites have evolved the ability to manipulate hosts. But I was disappointed to find no good examples of parasites that manipulate the sexual behavior of their hosts. In fact, female rats have actually evolved to steer clear of male rats infected with some other parasites. They can detect these infections even when the male rats look healthy, and they avoid these males to avoid getting sick. Now Vyas’s research suggests that there is at least one parasite that manipulates sex. Toxoplasma may be exquisitely unusual among parasites. But it’s also possible that there are other sex-hijacking creatures lurking out there. As for what this means for humans, I should point out there’s zero evidence of it moving from person to person, nor is there any evidence of it affecting the sexual behavior of humans. Then again, nobody has looked. For now, you can just let your inner Cronenberg take matters from here….

This came up this year at the science writers’ meeting, in the talk by Rob Knight, I think. And Robert Sapolsky talked about it a few years ago when we were at Stanford. He was devoting a lot of time to the study of rats and cats then, and he might well have some new stuff, the next time you visit Toxoplasma.

Ahhh, but then, if you get too much Toxoplasma, then you turn into a Crazy Cat Lady (or Guy), and the sex appeal goes *poof*. 😀 It would be interesting to see what would happen with a T. sample from a cat hoarder versus a T. sample from a “normal” cat lover.

Does T. gondii also reproduce sexually in larger cats (tigers, lions, pumas, cheetas etc, and perhaps dead species like the saber-toothed tiger)? I was thinking that if so, then there’s the possibility that, since people are infected by T. gondii, and they have some symptoms (not quite clear on what has been definitely established, slow motor function?), there could have been active selection on T. gondii to manipulate people as well to be better preyed upon by larger cats.

Maybe the parasite in a human makes that human like cats. Especially female humans. Think of all the human females who crave ownership of multiple cats; acquire ridiculous numbers of cats; melt, babbling, at icanhascheesburger.com pictures of cats. Thus, those human females are unwittingly spreading Toxoplasmosa, tra la, tra la, far and wide. (I did, in fact, test positive during pregnancy and, only with great self-denial, keep my personal cat population in single digits, but am non-libidinous, probably already having inadvertently infected my own offspring, who have multiple cats of their own. Never ending.)

Hi Carl, this is kind of off-topic but I wanted to bring it to your attention, in case you found it worth promoting: http://rockethub.com/projects/3737-support-zombie-research
She’s raising funds to do further research into zombies (also known as fish brain infecting parasites).
Oh, and the latest radiolab segment was really wonderful. Thank you for sharing such a personal story.

Seems like it wouldn’t be too hard to find a large-enough sample of young, recently-married human couples where the women grew up in cat-free environments (maybe allergic to cats) while the men had extensive exposure to cats, and then test the women.

One thing I’ve wondered about is how long, really, will the most of the parasites remain alive in people following exposure (and potentially affecting personalities). I would assume there’d by some kind of half life, that these things designed to live a rodent with a two-year lifespan can’t last forever, even if a few do last decades.

when my sister was pregnant with her first child, i warned her about this, but her dr said it wasn’t dangerous to the fetus. iwas soo pissed! not to mention that the worthless sperm-donor was too lazy and self-absorbed to clean the litter box for her…luckily,all went well. even go rid of the ‘father’ eventually.

In a related story, graduate students at Johns Hopkins University are on the trail of a related species of Toxoplasma that conceals itself by infecting and modifying the behavior of medical journal referees to make them less likely to publish articles containing evidence of its existence.

I wonder what the mechanism could possibly be? I mean, I’m not saying evolution produces perfection in all cases, but I do think there’s a pretty strong evolutionary incentive to increase sexual attractiveness. So, I would imagine, in a normal rat, there’s not a whole lot that can be done to increase sexual attractiveness without significantly reducing some other important ability (otherwise, a mutation that causes this increase would very, very quickly spread through the whole population).

So, I wonder a) how exactly the infected rats become more attractive, and b) what the infected rats have to give up to do so?

Q,
I don’t follow your logic. Why would an increase in sexual attractiveness require a loss of some beneficial genetic trait? There’s no reason to believe the increased attraction is a result of a mutation that would be passed to offsrping; it simply appears to be a consequence of the infection.

I’m hoping that answering your question a) is part of the ongoing research. They can’t leave us hanging like this